In physics, liquid is one of the states of matter. As is the case of a solid's molecules, the molecules of a liquid are able to move freely. Unlike gas molecules, molecules in a liquid often stay together. A substance can exist in each of these three states at different temperatures. For example, water is a liquid but it turns into steam, which is a gas, when it is placed under high temperatures. At low temperatures, water turns to ice, which is a solid.